1. Field of the Invention
This invention relates to an improved clutch facing, and more particularly to a clutch facing of the type used in motor vehicles. The invention also relates to an improved process for producing clutch facings of this type and to improved property modifiers for use therein.
2. Description of the Prior Art
Clutch facings of the type used in motor vehicles are generally composed of friction materials which comprise a relatively large porportion of asbestos fiber, together with other materials known as property modifiers which are added to provide desired levels of friction, wear, noise, and other properties of the friction material, and a resin binder which binds the other ingredients together as a single shaped article. Particular property modifiers include cured resinous friction dust derived from cashew nutshell liquid, ground rubber, and various carbonaceous materials such as carbon black, petroleum coke flour, and graphite. A common type of clutch facing for use in automobiles is prepared from asbestos fibers, cashew resin friction dust, graphite, and an organic resin binder.
Clutch facings for automobile use are generally prepared by the following process:
(1) An asbestos cord is prepared by spinning asbestos fibers about a fine brass wire which serves as a core. The friction property modifiers are incorporated into the spun asbestos cord as it is prepared.
(2) The asbestos cord is soaked in a binder such as a thermosetting phenolic resin.
(3) The resin-impregnated cord is dried.
(4) A green or uncured clutch face is then formed by placing strands of the asbestos cord side-by-side to form a sheet of the desired shape. This is often done simply by winding the asbetos cord in a tight spiral pattern on a flat surface.
(5) The uncured clutch face is then molded under heat and pressure to cure the clutch face in the desired shape. The molded clutch face is then cooled.
(6) The surface of the cured clutch facing is then ground smooth to produce a finished clutch facing.
The friction property modifiers most commonly used in automobile clutch facings are cashew resin friction dust and graphite. The combination of these two friction property modifiers has been found to produce especially good results in clutch facings of this type. These modifiers are incorporated into the asbestos cord when it is spun. However, the incorporation of the graphite into the asbestos cord at this step in the process has been founsd to present some problems. Since the graphite is in the form of a very finely divided and very light powder, it is difficult to keep it properly contained while being mixed with the asbestos fiber. This causes severe environmental problems in the factory, on the one hand, and on the other leads to an asbestos cord which may not contain the proper amount of graphite powder, and a clutch facing which, accordingly, does not have the desired properties.
In order to overcome this problem, an improved method has been proposed wherein a suitable amount of graphite powder is incorporated in the liquid uncured binder, and the asbestos fiber cord is soaked in this liquid binder. However, the clutch facings prepared using this impregnated asbestos cord have not been entirely satisfactory. The problems arise because the graphite powder is not uniformly incorporated into the asbestos fiber cord, but rather is concentrated on its surface. This results in a poor bond between cords which allows adjacent cords to be easily peeled apart. This peeling of adjacent cords is considered to be a very serious defect in a clutch facing.
Accordingly, a need has continued to exist for a clutch facing having a graphite friction property modifier uniformly distributed throughout, which is free of the problems of peeling of adjacent cords, and which can be manufactured by a process which avoids the environmental problems of incorporating graphite powder into the spun asbestos cord.